In this work, we investigated the change in the hydraulic conductivity of a sandyloam soil as a function of SAR and electrolyte concentration of the percolating solutions. High sodium adsorption ratios and low electrolyte concentrations caused a progressive reduction in the percolating rate, resulting in a non-linear relationship between effluent volume and time. In all these cases (nonsteady-state drainage) Darey's Law, which characterizes steady or stationary flow processes, does not apply. The relationships between effluent volume and time were then analyzed to find an empirical equation for predicting drainage from soil columns under saturated conditions. Characteristic parameters of the proposed model are the saturated hydraulic conductivity at time t = 0 (Ksat) and a rate constant that takes into account the progressive reduction in the initial hydraulic conductivity of the flow process. Validation of the model was found by applying it to montmorillonite-sand systems.